1. Field of the Invention
The present invention relates to a method, apparatus, and program for processing tomographic images, that generates optical tomographic images by OCT (Optical Coherence Tomography) measurement. The present invention also relates to a tomographic imaging system that employs the method, apparatus and program for processing tomographic images.
2. Description of the Related Art
Conventionally, there are cases in which optical tomographs that utilize OCT measurement are employed to generate optical tomographic images of living tissue. In these optical tomographs, a low coherence light beam emitted from a light source is divided in to a measuring light beam and a reference light beam. Thereafter, a reflected light beam, which is the measuring light beam reflected by a measurement target when the measuring light beam is irradiated onto the measurement target, is combined with the reference light beam. Tomographic images are obtained, based on the intensity of a coherent light beam obtained by combining the reflected light beam and the reference light beam. There are some optical tomographs that utilize TD-OCT (Time Domain Optical Coherence Tomography) measurement. In TD-OCT measurement, the measuring position in the depth direction (hereinafter, referred to as “depth position”) within a measurement target is changed, by changing the optical path length of the reference light beam.
Recently, OCT apparatuses that generate optical tomographic images at high speeds without changing the optical path length of the reference light beam, by utilizing FD-OCT (Fourier Domain Optical Coherence Tomography) measurement, have been proposed. SD-OCT (Spectral Domain Optical Coherence Tomography) measurement and SS-OCT (Swept Source Optical Coherence Tomography) measurement are two types of FD-OCT measurement (refer to Japanese Unexamined Patent Publication Nos. 2006-132996 and 2005-283155, U.S. Pat. No. 5,956,355, and Y. Yasuno et al., “Three-dimensional and high-speed swept-source optical coherence tomography for in vivo investigation of human anterior eye segments”, OPTICS EXPRESS, Vol. 13, No. 26, pp. 10652-10664, 2005). In SD-OCT measurement, low coherence light beam having a predetermined wavelength band is divided into a measuring light beam and a reference light beam by a Michelson interferometer, to obtain tomographic images. In SS-OCT measurement, the frequency of a laser beam emitted from a light source is swept. Reflected light beams of each wavelength are caused to interfere with the reference light beam. The intensities of reflected light beams at a depth positions within a measurement target are obtained by administering Fourier analysis on interference spectra for the series of wavelengths. The tomographic images are obtained employing the detected intensities.
In the SS-OCT measurement, the swept wavelengths of the laser beam fluctuate, and therefore the timing for a single period of wavelength sweeping is not always constant. Therefore, performing signal conversion of observed interference signals such that the interference signals are equidistant from each other with respect to wave numbers k has been proposed in U.S. Pat. No. 5,956,355.
It is necessary for the interference signals to be equidistant from each other with respect to wave numbers k, in order to obtain reflection data regarding a predetermined depth position by spectrally analyzing the interference signals. In the case that the interference signals are not arranged equidistantly with respect to the wave numbers k, the resolution of a tomographic image obtained therefrom deteriorates. In order to suppress this deterioration in resolution, the tomograph disclosed in U.S. Pat. No. 5,956,355 stores the wavelength sweeping properties of the light source unit in advance, and rearranges the interference signals such that they become equidistant with respect to the wave numbers k, based on the wavelength sweeping properties.
U.S. Pat. No. 5,956,355 presumes that the swept wavelengths do not change linearly with respect to time, and that the manner of change is reproducible, when rearranging the data string of the interference signals such that they become equidistant with respect to the wave numbers k. However, it is difficult for a light source unit to sweep wavelengths with the same wavelength sweeping properties for every period, and there are cases in which the wavelengths fluctuate for each period, due to the operating environment and the like. In the case that the fluctuation in wavelengths is present, the aforementioned signal conversion is not capable of rearranging the interference signals such that they are equidistant with respect to the wave numbers k, which results in deterioration of the resolution of tomographic images.